1. FIELD OF THE INVENTION
This invention relates to a system for driving a brushless motor without the use of a rotor position detector such as a Hall element which detects the position of the rotor of the motor.
2. DESCRIPTION OF THE PRIOR ART
Among various kinds of drive motors, a brushless motor has recently been most frequently employed, because the brushless motor has various advantages including a long useful service life, a high reliability and a decreased size and shape. However, a rotor position detector such as a Hall element is commonly required for detecting the position of the rotor of the brushless motor, and a so-called commutation sensorless brushless motor which does not require such a rotor position detector has been demanded for the purpose of further decreasing the cost, size, etc. of the brushless motor.
A prior art system for driving such a brushless motor is disclosed in, for example, JP-A-52-80415.
The prior art, brushless motor drive system disclosed in the publication cited above will now be described with reference to FIG. 13 which is a circuit diagram of the prior art drive system.
Referring to FIG. 13, drive coils 1, 2 and 3 are common-connected at one end thereof. The drive coil 1 connected at the other end thereof to the anode of a diode 4, to the cathode of a diode 5 and to the collectors of drive transistors 10 and 13. The drive coil 2 is connected at the other end thereof to the anode of a diode 6, to the cathode of a diode 7 and to the collectors of drive transistors 11 and 14. The drive coil 3 is connected at the other end thereof to the anode of a diode 8, to the cathode of a diode 9 and to the collectors of drive transistors 12 and 15. The cathodes of the diodes 4, 6, 8 and the emitters of the drive transistors 10, 11, 12 are connected to a positive power supply line connected to V.sub.cc, the anodes of the diodes 5, 7, 9 and the emitters of the drive transistors 13, 14, 15 are grounded. The drive coils 1, 2 and 3 are also connected at the other end thereof to a filter circuit 16, and output signals F.sub.1, F.sub.2 and F.sub.3 of the filter circuit 16 are applied to an energization switching circuit 17. Output signals U.sub.H, V.sub.H, W.sub.H, U.sub.L, V.sub.L and W.sub.L of the energization switching circuit 17 are applied to the bases of the drive transistors 10 to 15 respectively.
The operation of the prior art brushless motor drive system having the construction described above will now be described with reference to FIG. 14.
In FIG. 14, symbols U.sub.O, V.sub.O and W.sub.O designate waveforms of energization signals supplied to the drive coils 1, 2 and 3 respectively. These energization signal waveforms U.sub.O, V.sub.O and W.sub.O are supplied to the filter circuit 16 which removes higher harmonics from these waveforms and acts to delay their phases by 90.degree. relative to each other, so that the respective output signals F.sub.1, F.sub.2 and F.sub.3 having waveforms as shown appear from the filter circuit 16. This filter circuit 16 is a primary filter such as, for example, an RC passive filter or a primary Miller integration circuit, and its cut-off frequency is set to be sufficiently low as compared to that of the drive-coil energization signal waveforms. The output signals F.sub.1, F.sub.2 and F.sub.3 of the filter circuit 16 are converted into the signal waveforms U.sub.H, U.sub.L, V.sub.H, V.sub.L, W.sub.H and W.sub.L by logical processing in the energization switching circuit 17, and these signal waveforms U.sub.H, U.sub.V, V.sub.H, V.sub.L, W.sub.H and W.sub.L are used for switching the drive transistors 10 to 15. In this case, the switching operation of these drive transistors 10 to 15 is such that the motor drive torque is always produced in one direction only thereby rotating the brushless motor in that direction.
However, in the prior art drive system described above, the filter circuit 16 is required to have low-range cut-off frequency characteristics for the individual phases of the drive coils 1 to 3. Therefore, many capacitors having large capacity are required.
Also when the drive coils have large inductance, there is a tendency of occurrence of so-called armature reaction in which, after the drive transistors 10 to 15 are turned on, energization currents for energizing the drive coils 1 to 3 are supplied with a delay time.
In such a case, it is known that the efficiency is lowered when the drive coils 1 to 3 are energized with the timing shown in FIG. 14. With a view to improving the lowered efficiency described above, JP-A-52-80415 cited already proposes a method in which the phases of the output signals F.sub.1, F.sub.2 and F.sub.3 of the filter circuit 16 are somewhat advanced so as to operate the drive transistors 10 to 15 with such advanced phases thereby compensating the delayed energization attributable to the armature reaction. However, parts including additional capacitors are further required so as to effect such compensation. Further, because the drive-coil energization signal waveforms U.sub.O, V.sub.O and W.sub.O include spike noise generated at the turned-off time of each of the drive transistors 10 to 15 in addition to current variations due to power supply voltage variations and load variations, it is frequently difficult to accurately derive the energization switching signals U.sub.H to W.sub.L from the drive-coil energization signal waveforms U.sub.O, V.sub.O and W.sub.O by the use of the filter circuit 16. With a view to dealing with such a difficulty, a method as disclosed in Japanese Patent Publication No. 59-36519 was proposed.
However, the method of deriving the energization switching signals from the drive-coil energization signal waveforms by the use of the filter circuit has basically a problem as pointed out now. That is, a voltage drop due to the supply of the energization currents during energization of the drive coils and a voltage drop due to the internal impedance of the drive coils, as well as spike noise generated immediately after cessation of the drive-coil energization, are superposed on the fundamental wave (the counter-electromotive voltage) of the drive-coil energization signal waveforms, and these factors incessantly fluctuate with power supply voltage variations and load variations. Therefore, when the drive-coil energization signal waveforms are filtered to obtain the energization switching signals, there occurs an error attributable to the above-described factors superposed on the fundamental wave (the counter-electromotive voltage) of the drive-coil energization signal waveforms while incessantly fluctuating, resulting in difficulty of accurate energization of the drive coils.
Various methods have hitherto been proposed so as to solve the problem pointed out above thereby providing energization switching signals that can effect accurate energization of the drive coils. These prior art methods are basically such that the filter circuit and associated parts are suitably adjusted so as to maintain a fixed phase difference between the counter-electromotive voltages induced in the drive coils and the energization switching signals. However, this adjustment is very troublesome and time-consuming. Further, many capacitors are additionally required besides those constituting the filter circuit. Therefore, when it is desired to use the filter circuit and associated parts to form an integrated circuit, an increased number of externally mounted parts and pins are required, resulting in an expensive IC. On the other hand, JP-A-61-293191 describes a method in which the filter circuit is eliminated, and a microcomputer is used for digital processing so as to provide the desired energization switching signals. However, the disclosed arrangement is also expensive.
It will thus be seen that, in the prior art, brushless motor drive systems, energization switching signals having a fixed phase relative to the position of the rotor of the motor are derived by a filter circuit from drive-coil energization signal waveforms, and such energization switching signals are utilized to successively energize the drive coils. Therefore, it is difficult to produce accurate energization switching signals due to adverse effects including spike noise contained in the drive-coil energization signal wave-forms, a voltage drop across the drive coils due to flow of energization currents, fluctuations of those superposed factors due to power supply voltage variations and load variations, and armature reaction. Further, many capacitors having a large capacity are required to constitute the filter circuit. This is disadvantageous from the aspect of costs especially when the filter circuit and its associated parts are used to form an IC, because an increased number of externally mounted parts and pins are required.
Further, when the energization switching pulse signals are supplied so as to selectively energize the drive coils, noise (referred to hereinafter as electromagnetic noise) is generated due to attraction and repulsion between the rotor magnet and the stator coils. Thus, the prior art brushless motor drive systems had various problems as pointed out hereinbefore.